Expose Unit Spawner Attributes

[Muppet9010]

Could the unique attributes of the Unit Spawner prototype be exposed as read only within the API under LuaEntity.

It was the "call_for_help_radius" attribute that I was trying to get, but after a discord discussion with Bilka I also notice that a number of other Unit Spawner specific attributes aren't exposed, i.e. spawning_radius

[Bilka]

Added LuaEntityPrototype::call_for_help_radius, max_count_of_owned_units, max_friends_around_to_spawn, spawning_radius and spawning_spacing read for the next version.